In television receivers a down-conversion is carried out by heterodyning the incoming radio-frequency (RF) signals of different transmission channels with the oscillations of a tunable-frequency oscillator, thereby to generate lower-in-frequency radio-frequency signals within an intermediate-frequency (IF) band which is selected and amplified in an intermediate-frequency (IF) amplifier. An intermediate-frequency amplifier for the video portion of a television signal is commonly termed the "PIX IF amplifier". An intermediate-frequency amplifier for the sound portion of a television signal may be separate from the PIX IF amplifier or, alternatively, may include the PIX IF amplifier as is the case in TV sets of the intercarrier sound type. A PIX IF amplifier is typically required to handle signals ranging from about 50 microvolts to about 100 millivolts RMS. This represents a dynamic range of about 66 dB.
Within this specification the term "RF signal" shall be used in reference to signals at points in a television receiver before the down-conversion, or first detection; and the term "IF signal" shall be used in reference to signals at points in a television receiver after the down-conversion, or first detection, and before video detection, or second detection. In the claims following this specification, however, the term "RF signal" shall be used in reference to signals at all these points in a television receiver; and the term RF amplifier is to be construed as comprising IF amplifiers as well as other types of RF amplifier.
In providing an automatic gain control (AGC) function, it is desirable that certain operating conditions be met for each amplifier stage or device. Thus, the input signal level should exceed the internal noise by a predetermined factor, and the input signal level should not overload the device and thereby cause signal distortion and a bias shift. Furthermore, the AGC control signal should not itself cause undesirable bias shifts and thereby cause devices to be shifted from their intended operating points. E.g., the operating points for amplifiers and mixers are chosen to provide for low distortion in their output signals, and the operating points for mixers and detectors are chosen to provide for relatively high second-order responses.
At relatively strong signal levels in the order of 1 millivolt or more, it is particularly important that the gain be controlled in a manner that takes into account the so-called "noise/overload window". If, on the one hand, the gain of an earlier stage of a multiple-stage amplifier is reduced too little, overload with distortion undesirably may occur in a later stage. If, on the other hand, if the gain in an earlier stage is too low, thermal noise may become noticeable. It is desirable that a substantially noise-free and undistorted picture be achievable for an input signal level corresponding to 10 millivolts or so, measured at typical impedance levels. If an amplifier exhibits an inadequate noise/overload window, it may contribute noise or overload distortion at signal levels where a low distortion, relatively noise-free picture should be possible.
The development of integrated-circuit (IC) gain blocks spurred the need for block filtering. Recent practice has been to implement the IF filtering and gain functions in TV receivers in the configuration of a block filter followed by a gain-block IC amplifier. A surface-acoustic-wave (SAW) filter can provide the entire passband shape and adjacent channel attenuation required by a television receiver. Additional information on SAW filters and on block filtering and amplification may be found, for example, in Chapter 13 of the book TELEVISION ENGINEERING HANDBOOK; K. Blair Benson, Editor in Chief; McGraw-Hill Book Company, New York; 1986.
While the advent of block filtering and amplification has been generally desirable in the TV receiver art, it has nevertheless aggravated the problem of the noise/overload window for a number of reasons. Typical commercially available SAW filters used as a lumped filter at the input of an IF amplifier exhibit high insertion loss and high impedance, thereby acting as a relatively high level noise source impedance. The noise margin side of the noise/overload window is thereby reduced. Furthermore, noise signals falling within +/- 4.5 MHz of the picture carrier will be demodulated as noise that is "folded" into the 0-4.5 MHz video band.
This arises as follows. The IF signal lies within the band of 41.25-45.75 MHz. With the use of lumped or block filtering at the input of an IF amplifier, the sideband noise of IF stages following the filter is not suppressed as was the case when filtering was distributed stage by stage. This is because the noise within the band of +/- 4.5 MHz centered about the (IF) picture carrier frequency of 45.75 MHz is not filtered by the lumped filter ahead of the amplifier.
Another effect which tends to aggravate the noise/overload window problem in the block filtering and amplification procedure is that the typical bipolar IC amplifier utilized exhibits a transfer characteristic having a fixed overload voltage level, which restricts the overload side of the noise/overload window. Furthermore, typical modern small geometry bipolar transistors tend to exhibit a high base access resistance (r.sub.b) and hence tend to have a worse noise figure than larger, optimized devices having low r.sub.b ; this aggravates the problem.
The inventors recognize that the noise/overload window can be extended on the overload side by utilizing transistors of different design and, on the noise side by transforming the SAW filter output impedance to a lower value, thereby reducing its contribution as a noise source. However, impedance matching arrangements such as transformers or other matching circuits are costly, bulky, and raise the gain requirements on a system that already has high gain.
The problem of the noise/overload window is further complicated by the fact that each of certain ones of prior-art gain-controlled IF amplifiers exhibits a shift of its output bias voltage as a function of gain control. Generally, this results in a change of bias voltage on the demodulator, which is typically direct coupled to the IF amplifier. As was mentioned above with regard to operating points, such change is undesirable. As a result of the shifting bias conditions, adequate bias voltage must be provided to accommodate the changes, thereby complicating the demodulator design and necessitating a higher supply voltage than is otherwise required for low distortion.
A basic amplifier stage that is often used in IF amplifiers is the long-tailed pair or emitter-coupled differential amplifier, which includes two transistors with a "tail" connection between their emitter electrodes to which a constant current generator connects. The constant current generator may be provided by a high-resistance resistor between the tail connection and a remote direct potential; but in ICs, where there is a desire to use smaller operating potentials in order to keep dissipation within acceptable bounds, the constant current generator is generally provided by the principal conduction path of another transistor biased for constant current operation. While the long-tailed pair is oftentimes referred to as an emitter-coupled "differential" amplifier, in fact, it is often operated with a single-ended input circuit, a single-ended output circuit, or both. It is recognized herein that gain control may be effected by the straightforward reduction of the operating or tail current of an emitter-coupled differential amplifier, thereby reducing its mutual conduction in a known manner. However, there are drawbacks to the simple application of this approach. First, the noise source resistance is increased as gain decreases, thereby negating to an extent the improved signal-to-noise ratio associated with a larger signal and second, the power handling capability is reduced when it is most needed to handle a larger signal.
Prior-art untuned amplifiers, constructed in IC form and used after block filtering for television-receiver IF amplification in commercially successful TV receiver designs, have used three successive gain-controlled stages in order to meet the dynamic range requirements of about 66 dB for such service. These designs have used reverse AGC, in which the transconductances of the amplifier transistors are reduced in order to effect gain reduction. The voltage gain of an undegenerated common-emitter transistor amplifier is g.sub.m R.sub.L, where g.sub.m is the transconductance of the transistor and R.sub.L is the resistance of the collector load used with the transistor. The reduction of the transconductances of the amplifier transistors raises the resistances of the noise sources presented to their collector electrodes, increasing the thermal noise generated by the transistors and thus making it necessary to use three successive gain-controlled stages in order to keep the overall noise figure for the PIX IF amplifier chain low enough to meet commercial requirements. An alternative approach for reducing the gains of cascaded amplifier stages is to reduce the collector resistances used with the transistors, the well known forward AGC being an example of this approach. If the transconductances of the transistors are not reduced, there is no attendant increase in the thermal noise generated by the transistors; and reducing the collector resistances used with the transistors reduces the voltages associated with the currents generated by their thermal noise.
The inventors sought two-stage untuned amplifiers constructed in IC form that could, without need for unit-by-unit adjustments, reproducibly provide television-receiver IF amplification with 66 dB dynamic range. Reduction in the number of gain-controlled amplifier stages was sought in order to reduce power supply requirements, including power supply decoupling filtering between voltage-amplifier stages; to reduce the likelihood of self-oscillatory tendencies in portions of the gain control range owing to excessive phase shifting through the amplifier chain at frequencies where voltage gain was still appreciably large; and to simplify automatic-gain-control (AGC) tracking among the gain-controlled amplifier stages in the IF and RF portions of the television receiver. To reduce self-oscillatory tendencies in portions of the gain control range still further, the inventors perceived, it is desirable to have controlled-gain amplifiers that are quite symmetrical in structure so that balanced signal operation may be pursued, especially further on in the amplifier chain, and so that the amplifier chain can be laid out symmetrically on the IC. These steps reduce the positive feedback via stray capacitance from later to earlier portions of the amplifier chain.